Optical information processing equipment and semiconductor light emitting device suitable therefor

ABSTRACT

An information processor of a high reliability and a high recording density, and a blue color, blue-violet color and violet color based semiconductor light emitting device operable at a low threshold current density, used for the same, are provided. An optical information processor of a high reliability and a high recording density enables a moving picture, such as a high-definition television picture, to be recorded and reproduced satisfactorily. A barrier layer in a quantum-well active layer of a semiconductor light emitting device is doped with n-type impurities at a high density. Alternatively, the face orientation of a quantum-well active layer of a semiconductor light emitting device is a plane inclined from the (0001) plane, whereby the threshold current value of the semiconductor light emitting device can be decreased. The semiconductor light emitting device is typified by a gallium nitride based compound semiconductor laser device.

This is a continuation of U.S. application Ser. No. 09/297,147, filed onApr. 26, 1999 U.S. Pat. No. 6,542,526, which is a section 371 ofInternational Application No. PCT/JP97/03931, filed Oct. 29, 1997, andthe entire disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to optical information processingequipment such as an optical disk apparatus and a light source,particularly, a semiconductor light emitting device suitable therefor.

BACKGROUND ART

With regard to optical information processing equipment represented by aDVD (Digital Video Disk) apparatus, there have been strong demands toincrease the capacity thereof. In particular, it has been required toprovide optical information processing equipment capable of processingimages of high-definition TVs. As such information processing equipment,there has been generally known a type adopting a photomagnetic recordingor phase transition recording method. However, there has yet to berealized an optical recording equipment capable of sufficientlyprocessing dynamic images of high-definition TVs.

On the other hand, with regard to a semiconductor light emitting deviceused as a light source of such optical information equipment, it hasbeen required to shorten the wavelength toward a wavelength range fromthat of blue light to that of near ultraviolet light. The reason forthis is that a blue color based semiconductor light emitting device isuseful for high density recording or reproducing of information in orfrom a recording medium because such a light emitting device is capableof emitting light having a shorter-wavelength thereby focusing the lightat a smaller light spot.

Based on such a background, development has been actively made of a bluecolor semiconductor light emitting device using a gallium nitride basedcompound semiconductor such as GaN, GaAlN, InGaN or InGaAlN allowingemission of light having a wavelength in a wavelength range from that ofblue light to that of ultraviolet light.

At present, as a light emitting device using the above-describedmaterial, a high-intensity blue color LED having a double-heterostructure using a Zn-doped InGaN layer provided on a sapphire substrateas a light emitting layer has been put in practical use (S. Nakamura etal., Appl. Phys. Lett., 64 (1994) 1687). In recent years, continuousoscillation at room temperature of a semiconductor laser device using anundoped InGaN quantum-well layer provided on a sapphire substrate as alight emitting layer has been realized (S. Nakamura et al., Appl. Phys.Lett. 69 (1996) 4056) This semiconductor laser device, however, hasrequired a high operational current density, thereby failing to obtainsufficient characteristics from the viewpoint of practical use. Also, inrecent years, pulsive oscillation at room temperature of a semiconductorlaser using an undoped InGaN quantum-well layer provided on a sapphiresubstrate as a light emitting layer has been realized (S. Nakamura etal., Jpn. J. Appl. Phys. 35 (1996) L74). This semiconductor laserdevice, however, has also required a high operational current density,thereby failing to realize continuous oscillation thereof.

In the meantime, a semiconductor laser device including an active layerof a modulation doping structure, thereby allowing operation at a lowthreshold value has been disclosed in Japanese Patent Laid-open No. Sho62-249496.

A semiconductor light emitting device of a type using a gallium nitridebased material has been also reported, for example, in Jap. J. Appl.Phys. Vol. 35 (1996) Pt. 2, No. 1B, pp. L74-L76, or Japanese Patent Laidopen No. Hei 8-64912. The former document has disclosed a semiconductorlaser device having an InGaN based multi-quantum well structure, and thelatter document has disclosed a semiconductor light emitting deviceusing a gallium nitride based material. In the methods disclosed in boththe documents, the semiconductor light emitting device is configured byforming a wurtzite type gallium nitride based semiconductor layer on asapphire substrate. In addition, the active layer in the latter devicehas no quantum-well structure.

DISCLOSURE OF INVENTION

1. Information Processing Equipment

A first object of the present invention is to provide informationprocessing equipment capable of sufficiently recording or reproducingeven dynamic images in or from high-definition TVs or the like. Theoptical information processing equipment is typically configured as anoptical disk apparatus adopting, for example, photomagnetic recording orphase transition recording method.

A second object of the present invention is to provide opticalinformation processing equipment capable of ensuring recording orreproducing of high-definition dynamic images under variousenvironments, particularly, under a high temperature environment.

A third object of the present invention is to provide opticalinformation processing equipment enabling high density recording andprolonging the service life.

It should be noted that the optical information processing equipment ofthe present invention allows not only writing operation of informationbut also reading operation of information.

Principal aspects of the present invention disclosed in this Descriptionwill be described below in connection with the above-described objects.

(1) According to a first aspect of the present invention, there isprovided optical information processing equipment including at least alight source for irradiating a recording medium with light and anoptical system for focusing light on the recording medium, and having afunction of performing recording by changing a state of part of therecording medium, characterized in that a wavelength of light emittedfrom the light source is in a wavelength range of 350 nm to 550 nm, athreshold current density of the light source is in a range of 2.2kA/cm² or less, and a recording density of the recording medium is in arange of 15 GB or more. To achieve the above object, the thresholdcurrent density of the light source is preferably in a range of 1.0kA/cm² or less, more preferably, in a range of 0.8 kA/cm² or less.

The light source exhibiting a threshold current density in the aboverange is capable of sufficiently realizing optical output of 40 mW ormore at a drive current of 100 mA, for example, at an atmospherictemperature of 70° C.

The optical information processing equipment of the present invention issignificantly desirably usable while satisfying requirements ofrecording media such as high-definition TVs. In a high-definition TV, aninformation transfer speed for transferring a dynamic image is in theorder of, for example, 10 MB/sec. The known recording equipment hasfailed to record such a large capacity of information. On the contrary,according to the inventive equipment, since the recording density of arecording medium is in the range of 15 GB or more, the equipment enablesrecording of a dynamic image in a high-definition TV. Also, according tothe inventive equipment, since the light source allows emission of lightin the wavelength range of 350 nm to 550 nm, that is, emission ofvisible light, particularly, blue light, violet light or ultravioletlight, the above-described recording density can be realized on onesurface of the disk. Further, according to the inventive equipment, thethreshold current density of the light source is low and, therefore, itis possible to require less power and, hence, to reduce the powerconsumption. The threshold current density of the light source of theinventive equipment is substantially reduced to half as compared withthe general threshold current density of the conventional blue color orviolet color based semiconductor laser device. Accordingly, the lightsource of the inventive equipment is able to reduce the drive power andhence to meet the practical requirements of the present product market.

To achieve the above objects, the wavelength of light emitted from thelight source is preferably in a range of 350 nm to 430 nm. This isadvantageous in simplifying the optical system.

(2) According to a second aspect of the present invention, there isprovided optical information processing equipment including at least alight source for irradiating a recording medium with light and anoptical system for focusing light on the recording medium, and having afunction of performing recording by changing a state of part of therecording medium, characterized in that the light source is composed ofa semiconductor laser device allowing emission of light in a wavelengthrange of 350 nm to 550 nm and having a threshold current in a range of10 mA or less. It should be noted that the threshold current is thevalue measured at room temperature (20° C.).

Like the equipment according to the first aspect of the presentinvention, the optical information processing equipment according tothis aspect is significantly desirably usable while satisfyingrequirements of recording media such as high-definition TVs. Also,according to the equipment according to this aspect, since the thresholdcurrent of the light source is in the range of 1.0 mA or less, it ispossible to save the unavailable power and hence to reduce the powerconsumption. The threshold current density of the light source of theequipment according to this aspect is substantially reduced to half ascompared with the general threshold current density of the conventionalblue color based semiconductor laser device.

(3) According to a third aspect of the present invention, there isprovided optical information processing equipment including at least alight source for irradiating a recording medium with light and anoptical system for focusing light on the recording medium, and having afunction of performing recording by changing a state of part of therecording medium, characterized in that a wavelength of light emittedfrom the light source is in a wavelength range of 350 nm to 550 nm, athreshold current density of the light source is in a range of 2.2kA/cm² or less, a threshold current of the light source is in a range of10 mA or less, and a recording density of the recording medium is in arange of 15 GB or more. In particular, to achieve the above object, thethreshold current density of the light source is preferably in a rangeof 1.0 kA/cm² or less.

(4) According to a fourth aspect of the present invention, there isprovided optical information processing equipment including at least alight source for irradiating a recording medium with light and anoptical system for focusing light on the recording medium, and having afunction of performing recording by changing a state of part of therecording medium, characterized in that a wavelength of light emittedfrom the light source is in a wavelength range of 350 nm to 550 nm, arecording density of the recording medium is in a range of 15 GB ormore, and a drive current of the light source is in a range of 100 mA orless.

Like the equipment according to the first aspect of the presentinvention, the optical information processing equipment according tothis aspect is significantly desirably usable while satisfyingrequirements of recording media such as high-definition TVs. Also,according to the equipment according to this aspect, since the drivecurrent of the light source is low, it is possible to reduce the powerconsumption.

In the equipment according to this aspect, the threshold current densityof the light source is preferably in a range of 2.2 kA/cm² or less andthe threshold current of the light source is preferably in a range of 10mA or less. In particular, to achieve the above objects, the thresholdcurrent density of the light source is more preferably in a range of 1.0kA/cm² or less. It should be noted that each of the threshold currentdensity and threshold current is the value measured at room temperature(20° C.).

In addition, the threshold current density is more preferably in a rangeof 1.7 kA/cm² or less. The same is true for some inventions of thefollowing optical information processing equipment to be describedlater.

(5) According to a fifth aspect of the present invention, there isprovided optical information processing equipment described in any oneof the items (1) to (4), wherein the light source is composed of asemiconductor light emitting device including a light emitting regionmade from a compound semiconductor material having a hexagonal system.

In the optical information processing equipment according to thisaspect, since the light source allows emission of light having awavelength in the short-wavelength area of a visible light region andthe recording density of a recording medium is 15 GB or more, theequipment enables recording of a dynamic image in a high-definition TV.Further, since the semiconductor light emitting device includes a lightemitting region made from a compound semiconductor material having ahexagonal system, it is possible to reduce the threshold current valuethereof.

(6) According to a sixth aspect of the present invention, there isprovided optical information processing equipment described in the item(5), wherein the threshold current value is in a range of 100 mA orless, preferably, in a range of 70 mA. In particular, the thresholdcurrent value of the light source of the above optical informationprocessing equipment is more preferably in a range of 40 mA.

Further, the light source is preferably configured as a semiconductorlight emitting device having at least an active layer region having astrained quantum-well structure which is made from a wurtzite typesemiconductor material. The threshold current value of such asemiconductor light emitting device is specified to be in a range of 100mA or less, preferably, in a range of 70 mA or less. The active layerregion having a strained quantum-well structure makes it easy to achievea lower threshold current value. With this configuration, there can berealized optical recording equipment ensuring a recording density beingas large as twice or more that of the conventional equipment, a highreliability, and a high service life.

While the optical information processing equipment described in each ofthe items (1) to (6) is described in terms of the writing function ofinformation in a recording medium, the present invention can be alsoapplied to the above equipment in terms of the reading function ofinformation from a recording medium.

Even if the above equipment is required to exhibit both the writing andreading functions, it may be provided with only one semiconductor lightemitting device because the semiconductor light emitting device isusable not only for the writing operation but also for the readingoperation.

According to the inventive equipment, since the recording density of arecording medium is 15 GB or more, the equipment is able to process adynamic image of a high-definition TV or the like. In this case, thewriting operation requires an operational power in a range of about 20mW to 40 mW, particularly, in a range of about 30 mW to 40 mW. On theother hand, the reading operation requires an operational power in arange of about 3 mW to 10 mW, particularly, in a range of about 4 mW to5 mW. In addition, the setting of the operational power is dependent onthe light availability of a reading or writing optical system. Ingeneral, the availability of an optical system for writing is about 30%,and accordingly, if the operational power for writing is set at a valueranging from 20 mW to 40 mW, the optical output required for writing inthe film surface of a recording medium becomes a value ranging fromabout 0.9 mW to 1.2 mW. On the other hand, the availability of aread-only optical system is about 10%.

For example, in a next-generation DVD system using a light sourceallowing emission of light in a range of a blue light to ultravioletlight, a semiconductor laser device as the light source will require anoperational current of 100 mA or less and an optical output of 40 mW forwriting information in an optical disk. In the case of using a nitridebased semiconductor laser most expected as the above semiconductor laserdevice, the characteristic temperature and slope efficiency areestimated at 120 K and 0.4 W/A, respectively, and accordingly, to assurehigh temperature operation at 70° C., it may be desirable to set thethreshold current density in a range of 1.0 kA/cm² or less, preferably,in a range of 0.8 kA/cm² or less. At this time, in a high temperatureenvironment of 70° C., the optical output of 40 mW required fordetecting reflected light or recording information can be obtained at acurrent value of 100 mA or less. This makes it possible to realizeoptical recording equipment ensuring a recording density being as largeas twice or more that of the conventional: equipment, a highreliability, and a high service life, without changing the configurationof the existing system. Also, since the service life of a light emittingdevice is strongly dependent on the carrier density required foroperation, the reduction in operational current density is of courseeffective to prolong the service life of the light emitting device.

The optical information processing equipment of the present inventionwill be described later in detail by way of the following embodiments.The optical information processing equipment is represented by anoptical disk apparatus such as a DVD or compact disk apparatus, or alaser beam printer.

The various light sources suitable for the optical informationprocessing equipment of the present invention will be described indetail in the following item “2. Semiconductor Light Emitting Device”.

2. Semiconductor Light Emitting Device

A semiconductor light emitting device most suitable for theabove-described optical information processing equipment is of a typeallowing emission of blue light, violet light or ultraviolet light,which device is typically configured as a semiconductor laser device.Such a semiconductor light emitting device represented by asemiconductor laser device is capable of satisfying various requirementsof the above optical information processing equipment. It should benoted that the semiconductor light emitting device, such as thesemiconductor laser device, of the present invention can be applied notonly to the optical information processing equipment described in thisDescription but also to other general applications matched with thecharacteristics of the device, such as a wavelength of light emittedtherefrom.

At present, an attempt has been made to realize high density opticalinformation processing equipment using a gallium nitride basedsemiconductor light emitting device; however, it has failed to put suchequipment into practical use. A high operational current density of agallium nitride based semiconductor light emitting device is the maincause of the above unsuccessful attempt.

The high operational current density of a gallium nitride basedsemiconductor light emitting device results from the fact that aneffective mass of the valence band of the material (semiconductormaterial) is larger than that of a semiconductor material having azinc-blend type crystal structure such as GaAs. Also, in a wurtzite typesemiconductor, energy dispersion in the valence band is little changedeven by applying a biaxial strain within crystal planes of crystals in aquantum-well layer (semiconductor layer causing carrier recombinationconcerning light emission) which is generally stacked in parallel to the(0001) plane. As a result, in a GaN based semiconductor having awurtzite type crystal structure, it is not expected to reduce the statedensity due to epitaxial strain within the (0001) plane. Further, evenif a technology associated with a semiconductor light emitting devicemade from a semiconductor material having the zinc-blend type crystalstructure, for example, disclosed in Japanese Patent Laid-open No. Sho62-249496 is applied to the above GaN based semiconductor having thewurtzite type crystal structure, a sufficient effect of reducing theoperational current of the GaN based semiconductor is not obtained.

Accordingly, a fourth object of the present invention is to provide asemiconductor light emitting device allowing emission of blue basedlight, blue-violet based light, and ultraviolet based light and ensuringa low threshold current density. The reduction in threshold currentdensity reasonably leads to a reduction in operational current densityof the light source.

As the semiconductor light emitting device allowing blue based light,blue-violet based light and ultraviolet based light, there may be usednot only a semiconductor laser device, but also a semiconductor devicehaving a hetero junction such as pn-junction or pin-junction andallowing light emission, for example, a light emitting diode device orsuper luminescent diode device.

The above-described fourth object of the present invention can beachieved by semiconductor light emitting devices roughly classified intothree kinds different in standpoint. One invention relates to thecarrier density in a light emitting region, and another inventionrelates to selection of crystal planes of semiconductor crystals of thelight emitting region. The selection of crystal planes is furtherdivided into two forms. Accordingly, the semiconductor light emittingdevices of the present invention provided for achieving the fourthobject are roughly classified into three types.

The first type semiconductor light emitting device of the presentinvention is characterized in that an n-type impurity is doped in alight emitting region having a quantum-well structure at a density of5.0×10¹⁸ cm⁻³ to 1.0 ×10²⁰ cm⁻³. The density of an n-type impurity ispreferably in a range of 1.5×10¹⁹ cm⁻³ to 5.0×10¹⁹ cm⁻³.

The second type semiconductor light emitting device of the presentinvention is characterized in that the oriented plane of a crystalregion of a compound semiconductor having a hexagonal system, whichregion constitutes an active layer region of the semiconductor lightemitting device composed of layers made from compound semiconductormaterials, is a plane inclined from the (0001) plane. The inclinationangle is in a range of 70° to 90°.

The inclination angle of the crystal plane may have a deviation, thatis, a tolerance of 5° or less. The deviation, that is, tolerance of theinclination angle is not limited to the deviation of the inclineddirection of the crystal plane, but it means the deviation, that is,tolerance of the crystal plane itself.

The third type semiconductor light emitting device of the presentinvention is characterized in that the oriented plane of a semiconductorcrystal region constituting an active layer having a strained superlattice structure of the semiconductor light emitting device composed oflayers made from compound semiconductor materials having a hexagonalsystem is a plane inclined from the (0001) plane at an inclination anglein a range of 3° to 70°.

The above compound semiconductor materials used for the semiconductorlight emitting device of the present invention are represented bygallium nitride based compound semiconductor materials. The galliumnitride based compound semiconductors are classified into those having awurtzite type crystal structure and those having a zinc-blend typecrystal structure.

The configurations of the first type semiconductor light emitting deviceand second type light emitting device may be used singly or incombination. In each case, for example, when being configured as asemiconductor laser device, the semiconductor light emitting device iscapable of reducing the carrier density required for oscillation oflaser, although the degree of the effect is dependent on the form of theembodiment, thereby reducing the operational current density of thegallium nitride based semiconductor laser device. The third typesemiconductor light emitting device can be also combined with the firsttype semiconductor light emitting device.

Hereinafter, principal configurations of the semiconductor lightemitting devices disclosed in this Description will be described indetail.

2-1: Semiconductor Light Emitting Device of First Type

(1) As a first aspect of the first type semiconductor light emittingdevice, there is provided a semiconductor light emitting device composedof at least compound semiconductors, the device including at least afirst conduction type cladding layer; a second conduction type claddinglayer; and an active layer region having a quantum-well structure, whichlayer is put between both the cladding layers and which has a well layerand barrier layers each having a forbidden band width larger than thatof the well layer; characterized in that an n-type impurity is doped inthe active layer region having the quantum-well structure, and thecarrier density in the well layer is in a range of 5.0×10¹⁸ cm⁻³ to1.0×10²⁰ cm⁻³.

Here, it is important that an n-type impurity, typically, Si is doped inthe active layer region.

(2) The carrier density of the above n-type impurity is preferably in arange of 1.5×10¹⁹ cm⁻³ to 5.0×10¹⁹ cm⁻³.

(3) As a second aspect of the first type semiconductor light emittingdevice, there is provided a semiconductor light emitting devicedescribed in the item (1) or (2), wherein the above n-type impurity isselectively doped in the barrier layer of the quantum-well activeregion. The structure in which an impurity is selectively doped iscalled a modulation-doped structure.

Additionally, it may be most desirable that the above n-type impurity beselectively present only in the barrier layer. In this case, the welllayer does not substantially contain the impurity, and thereby it doesnot produce any unnecessary trap level. Accordingly, such a form isuseful for the light emitting characteristic. From the viewpoint ofpractical use, however, it is allowed that the n-type impurity may passthrough the barrier layer and be slightly distributed at the boundaryregion. Such an allowance of the n-type impurity is set in order to makeeasier the manufacture of the semiconductor light emitting device. Thequantum-well structure not modulation-doped with an impurity will bedescribed later.

The features of the first type semiconductor light emitting device ofthe present invention, classified into the above items (1) to (3), willbe described below in detail.

Description will be made by example of a semiconductor light emittingdevice including a light emitting region having a quantum-well structurein which a well layer is put between barrier layers each having aforbidden band width broader (larger) than that of the well layer.Examples of such a light emitting region structure include a singlequantum-well (SQW) structure in which one well layer is put between twobarrier layers, and a multi-quantum well (MQW) structure in which aplurality of well layers and a plurality of barrier layers arealternately stacked. As the multi-quantum well structure, a strainedmulti-quantum well structure and a strain compensated multi-quantum wellstructure may, of course, be adopted.

In the case of providing a light emitting region having a quantum-wellstructure (hereinafter, referred to as “quantum-well active layerregion”) for carrying out the present invention, as described above, itmay be desirable to selectively dope an impurity in the barrier layer ata high density. The features of such a quantum-well active layer regionwill be described below.

FIG. 1 shows a relationship between an injected carrier density and anoptical gain for a modulation-doped quantum-well structure. Morespecifically, FIG. 1 shows the dependence of the injected carrierdensity on the maximum gain for a so-called GaN/Al_(0.2)Ga_(0,8)Nmodulation-doped quantum-well structure in which GaN layers (thickness:5 nm each) and Al_(0.2)Ga_(0.8)N layers (thickness: 5 nm each) arealternately stacked. In this figure, a density Nd of an n-type impuritydoped in a barrier layer is taken as a parameter. Here, carriersreleased from the impurity in the barrier layer are all supplied to awell layer. At this time, the electric charge neutralization conditionis established throughout the quantum-well active layer, and therefore,if the thickness of the well layer is equal to that of the barrierlayer, a density n₀ of the carriers having been released from theimpurity and locally present in the well layer becomes equal to thedensity Nd of the impurity doped in the barrier (n₀=Nd). The case wherethe thickness of the well layer is different from that of the barrierlayer will be described later.

The results of FIG. 1 shows that as the density Nd becomes larger, thecarrier density (transparent carrier density) required for makingpositive the gain is shifted on the low injected carrier density side.Here, the wording “making positive the gain” means that the optical gainon the ordinate is made positive to the injected carrier density on theabscissa in FIG. 1. That is to say, as the intercept of each graph(rising rightwardly) on the abscissa becomes smaller, the transparentcarrier density becomes smaller to reduce the carrier density requiredfor light emission, that is, “threshold carrier density” in laseroscillation.

Based on the results shown in FIG. 1, a relationship associated with anet threshold current density Jnom is obtained as shown in FIG. 2. Inthis figure, a threshold gain gth (500 cm⁻¹, 1000 cm⁻¹, and 1500 cm⁻¹)is taken as a parameter. FIG. 2 shows a relationship between a donorconcentration and a nominal current density. From the results of FIG. 2,it is understood that an effect of reducing the threshold currentdensity appears by setting the density Nd of the modulation-dopedimpurity at a value ranging from 5×10¹⁸ cm⁻³ or more. Also, it isapparent that the threshold current density can be reduced to 80% orless of that obtained in the case of the undoped quantum-well structureby setting the density Nd of the modulation-doped impurity at a valueranging from 1.5×10¹⁹ cm⁻³ or more. However, when the density Nd of theimpurity modulation-doped in the barrier layer is more than 5×10¹⁹ cm⁻³,the crystallinity of the barrier layer is significantly degraded, andtherefore, it may be desirable that the density Nd of the impuritymodulation-doped in the barrier layer be equal to or less than 5×10¹⁹cm⁻³.

In general, with respect to a long-wavelength band n-type impuritymodulation-doped laser, it is known that the temperature characteristicis degraded by carrier leakage on the conduction band side. However,with respect to a nitride based semiconductor, it was confirmed thatsince the band offset on the conduction band side can be made large, thecurrent leakage can be suppressed to prevent degradation of thetemperature characteristic by setting a difference in band gap energybetween the quantum-well active layer and cladding layer at a valueequal to or more than 0.35 eV.

The above description is made by way of the example in which an impurityis selectively doped in the barrier layer; however, to simplify themanufacturing process, the impurity may be doped in both thequantum-well layer and barrier layer in consideration of the requiredperformance of the semiconductor light emitting device. In this case,the impurity may be partially doped in both the quantum-well layer andbarrier layer or may be doped over the entire area of the quantum-wellactive region. The doping of the impurity in the quantum-well layerslightly degrades the performances of the light emitting device in termsof the quantum level and quantum size effect in the quantum-well layer;however, it is apparent that the doping of the impurity in both thequantum-well layer and barrier layer exhibits the effect of reducing thethreshold carrier density as compared with the case where the entirearea of the quantum-well active region is not doped with the impurity.

The same effect can be obtained by making larger the thickness of abarrier layer than that of a quantum-well layer. The thickness of thebarrier layer may be set about twice that of the quantum-well layer fromthe viewpoint of the practical use. For example, in the case of theabove modulation-doped structure, if the density of an n-type impuritydoped in a barrier layer is set at 1.5×10¹⁹ cm⁻³ and the thickness ofthe barrier layer is set twice that of a quantum-well layer, the carrierdensity in the quantum-well layer reaches 3.0×10¹⁹ cm⁻³. The effect canbe obtained in the case where an impurity is doped in at least part ofeach of the barrier layer and quantum-well layer.

The above description is made by example of the light emitting region ofthe quantum-well structure having the well layers and barrier layers;however, the first type semiconductor light emitting device of thepresent invention can be applied to a semiconductor light emittingdevice of a type in which optical guide regions or clad regions aredirectly joined to an active layer (equivalent to the well layer) bymodifying the configuration thereof as follows:

The forbidden band width of a semiconductor layer forming an opticalguide region or clad region (or a stacked structure of the semiconductorlayers) is wider than that of an active layer, although the forbiddenband width of the optical guide region is narrower than that of thecladding layer. An n-type impurity is doped in part of the junctioninterface side of each optical guide region or clad region joined to theactive layer, to form a layer portion equivalent to the barrier layer.In a general semiconductor laser, there is adopted a configuration inwhich the conduction type of one of clad regions or optical guideregions disposed on both sides of an active layer is reversed to that ofthe other, or both the clad regions or optical guide regions are leftundoped.

On the contrary, according to one embodiment of the present invention,the areas doped with the n-type impurity are formed in both the cladregions or optical guide regions. Such a configuration is slightlydisadvantageous in terms of stimulated emission of laser light. However,the area doped with the n-type impurity is not required to be extendedthroughout each optical guide region or clad region but rather desiredto be restricted only at a portion on the side of the junction interfacewith the active layer. In particular, in the case of adopting theso-called modulation-doped structure in which an impurity is notintended to be doped in the active layer, it may be desirable tosuppress the thickness of the n-type impurity doped region at a valueless than 10 nm for increasing the modulation-doping effect. As aresult, there can be obtained an advantage that the release of carriersfrom the n-type impurity doped region in the active layer is promotedwhile the laser light stimulated emission effect is not obstructed.Further, the modulation-doping is effective to suppress separation ofcarriers in the active layer due to the piezoresistance effect, whichseparation is liable to be caused in a wurtzite type semiconductor lightemitting device.

2-2: Relationship between Wurtzite Type Crystal Structure and EnergyBand Structure

Prior to description of the ground for creating the structures of thesecond type and third type semiconductor laser devices of the presentinvention, there will be described a relationship between a wurtzitetype crystal structure and an energy band structure.

An image in which the oriented plane of a light emitting region(quantum-well active region) as the basic structure of the semiconductorlaser device is taken as a plane inclined 70° to 90° from the (0001)plane will be described with reference FIGS. 3 to 5 each of which is adiagram showing the crystal structure of a gallium nitride basedsemiconductor crystal. In addition, FIG. 5 is the diagram illustratingplane orientation indices of the <lattice> of the wurtzite type crystalstructure, that is, a hexagonal system.

FIG. 3 shows an arrangement of atoms in a gallium nitride basedsemiconductor crystal having the wurtzite type structure as one kind ofthe hexagonal system. In this figure, a hexagonal column portion definedby a thick broken line designates a unit cell of the crystal. Theconventional gallium nitride based semiconductor crystal is epitaxiallygrown with the (0001) plane shown by the hatch lines rising rightwardly(more specifically, plane parallel thereto) taken as the growth plane.On the contrary, in the structure of the second type semiconductor lightemitting device of the present invention, as is apparent from theabove-described inclination angle range, a crystal plane perpendicularto the (0001) plane or a crystal plane inclined ±20° (±25° including theabove-described tolerance) from the above upright crystal plane on the(0001) plane side is taken as the growth plane.

To be more specific, in the second type semiconductor light emittingdevice of the present invention, for example, the (1,0,−1,0) plane shownby the hatch lines rising leftwardly in FIG. 3, and a plane 1 parallelthereto are taken as growth planes in the light emitting region,particularly, the active layer. The above inclination angle range [only,a half (0 to +20°) range is shown] with respect to the (1,0,−1,0) planein the second type semiconductor light emitting device of the presentinvention is shown as a region surrounded by two thick one-dot brokenlines in FIG. 4. In addition, FIG. 4 is a diagram seen from an arrow Aof FIG. 3 (an image seen in the direction along the plane 1).

The ground for creating the configuration of the second typesemiconductor light emitting device of the present invention will bedescribed below by way of the example in which the (1,0,−1,0) plane istaken as the growth plane of the light emitting region.

FIGS. 6A to 6D each show an energy dispersion in the upper portion ofthe valence band in the case where the oriented plane of a quantum-welllayer (active layer) is taken as the (1,0,−1,0) plane. The strainedamounts in the states shown in FIGS. 6A to 6C are as follows:e_(a)=e_(c)=0% for the state shown in FIG. 6A; e_(a)=e_(c)=0.5%(compressive strain) for the state shown in FIG. 6B; ande_(a)=e_(c)=0.5% (extension strain) for the state shown in FIG. 6C.Here, e_(a) and e_(c) are strained amounts within a quantum-well planeand are defined by

e _(a)=(a−a ₀)/a

e _(c)=(c−c ₀)/c

where a and c are lattice constants of the well layer in thequantum-well active region along the a-axis and c-axis, respectively,and a₀ and c₀ are lattice constants of the well layer of thequantum-well active region along the a-axis and c-axis in the state inwhich no biaxial stress is applied to the well layer, respectively.

For comparison, the energy dispersion for the quantum-well layer inwhich the oriented plane is taken as the (0001) plane is shown in FIG.6D.

In FIGS. 6A to 6D, the abscissas kx, ky, and kz designate crystalorientations [1,0,−1,0], [−1,2,−1,0], and [0001], respectively. Thetransparent carrier density required for light emission of the opticaldevice is substantially dependent on one, called first sub-band, of aplurality of curves (indicating the band structures) which appear oneach graph. The sub-bands are ranked in the order of increasing of theenergy at the center (point of k =0) of each graph. In FIGS. 6A, 6B and6D, the curve HHI becomes the first sub-band, and in FIG. 6C, the curveLH1 becomes the first sub-band. The effect of the sub-bands lower thanthe first sub-band exerted on the transparent carrier density issubstantially negligible. The first sub-band exerts an effect on thethreshold carrier density as follows. First, an effective mass m* ofcarriers (positive holes in this case) is obtained by secondarydifferentiation of an energy E of the first sub-band with respect to thewave number k as expressed by the following equation (1):

m*=h/(2πd ² E/dk ²)  (1)

A state density D(E) of a two-dimensional system has a relationship withthe value m* thus obtained as expressed by the following equation (2):

D(E)=4πm*/(Lzh ²)  (2)

In the above equations, h designates the Planck's constant and Lzdesignates the thickness of the quantum-well layer. The transparentcarrier density is substantially dependent on the state density D(E),and accordingly, as is apparent from the equation (2), the reduction ineffective mass of carriers leads to a reduction in injected amount ofcarriers required for light emission, that is, threshold carrierdensity.

As is apparent from comparison between FIGS. 6A and 6D, in the casewhere the oriented plane of the quantum-well layer is taken as the(1,0,−1,0) plane, the symmetry of the crystal is lost, so that theeffective mass of the heavy positive hole band (HH1) shown as the firstsub-band is reduced to about two-third that in the case where theorientated plane is taken as the (0001) plane. Also as shown in FIG. 6B,in the case where the compressive strain (e_(a)=e_(c)) is applied to thequantum-well layer, the valence band is split so that the non-parabolicshape of the first sub-band in the upper portion of the valance band isimproved. As the degree of necking at the shoulder of the first sub-bandis reduced and the first sub-band becomes a curve similar to a parabola,the width of the gain spectrum becomes narrower, which results in anadvantage in increasing the gain. Further, as shown in FIG. 6C, in thecase where the extension strain is applied to the quantum-well layer,light positive hole band (LH1) is shifted upwardly to be taken as thefirst sub-band, with a result that the effective mass is increased. Theresult of applying each of the quantum-well layers thus examined to asemiconductor laser device is shown in FIG. 7. In this figure, the curvedesignated by [(1,0,−1,0) unstrained] is equivalent to the quantum-welllayer shown in FIG. 6A; the curve [(1,0,−1,0) 0.5% comp.] is equivalentto the quantum-well layer shown in FIG. 6B; the curve [(0001)unstrained] is equivalent to the quantum-well layer shown in FIG. 6D;and the curve [(0001) 0.5% comp.] is equivalent to a quantum-well layerin which 0.5% compressive strain is applied to the state of thequantum-well layer shown in FIG. 6D.

The data shown in FIG. 7 are obtained for a quantum-well active regioncomposed of quantum-well layers made from GaN and barrier layers madefrom Al_(0.2)Ga_(0.8)N. In this figure, like FIG. 1, the abscissadesignates the injected carrier density and the ordinate designates theoptical gain.

As is apparent from comparison between FIGS. 1 and 7, by combining thestructures of the first type and second type semiconductor lightemitting devices of the present invention with each other, the curveindicating the optical gain increasing linearly with the injectedcarrier is further shifted on the left side. This means that thethreshold carrier density is further reduced. Further, as describedabove, the doping of the impurity in the active layer region iseffective to suppress separation of carriers due to the piezoresistanceeffect which is liable to be caused in a wurtzite type strainedquantum-well.

In summary, by providing a laser including the quantum-well layer havingthe (1,0,−1,0) growth plane, to which no strain or a compressive strainis applied, or the quantum-well layer having (grown along) an orientedplane: equivalent thereto in terms of energy, it is possible to reducethe state density in the upper portion of the valence band, and hence torealize laser oscillation at a low threshold value.

Here, the oriented plane equivalent to the (1,0,−1,0) plane in terms ofenergy means any one of all crystal planes perpendicular to the (0001)plane. That is to say, in the configuration of the second device of thepresent invention, plane indices (called Miller indices) of the planeshaving the above inclination angle of 90° are not limited. The reasonfor this is due to the isotropy of energy within a kx-ky plane in thewave number space (specifically, the (0001) plane or a plane parallelthereto) in a semiconductor material having the wurtzite type crystalstructure. For example, the (1,1,−2,0) growth plane is equivalent to the(1,0,−1,0) growth plane in terms of energy.

Next, the ground why the inclination angle is restricted in the range of3° to 70° or the range of 70° to 90° will be described.

FIG. 8 shows the results of theoretically calculating a relationshipbetween an inclination angle and an effective mass of positive holes inthe case where the growth plane of an active layer is inclined from the(0001) plane to the (1,0,−1,0) plane. The curve designated by“unstrained” shows data obtained in the case of the quantum-well activelayer region, and the curve designated by “strained” shows data obtainedin the case of the strained quantum-well active layer region. In otherwords, the curve “unstrained” shows the data in the case ofe_(a)=e_(c)=0. As shown in FIGS. 6A and 6B, the state of the bandstructure in the case of e_(a)=e_(c)=0 is substantially similar to thatin the case of e_(a)=e_(c)=compressive strain. Accordingly, it may beconsidered that the dependence of the inclination angle on the effectivemass in the case where the quantum-well active layer is left unstrained(e_(a)=e_(c)=0) is similar to that in the case where the quantum-wellactive layer has the compressive strain (e_(a)=e_(c)=compressivestrain). In the strained quantum-well structure shown in FIG. 8, thelattice unconformity between the well layer and barrier layer is 1.0%along the a-axis and 1.6% along the c-axis, that is, the compressivestress is applied to the well layer. To be more specific, the curve“strained” shows the data for the strained quantum-well structure havingthe compressive strain of e_(c)=1.6% and e_(a)=1.0%.

The results of FIG. 8 shows that in the case of “unstrained”, theeffective mass at an inclination ranging from 70° to 90° becomestwo-third that at an inclination of 0°, that is, in the case of (0001)plane; while in the case of “strained”, the effective mass at aninclination ranging from 10° to 70° becomes one-second to two-third thatat an inclination of 0°. Further, as is apparent from FIG. 8, in thecase of the strained quantum-well structure, the effective mass ofpositive holes at an inclination angle near 40° becomes one-second thatat an inclination angle of 0°, that is, in the case of the (0001) plane.

Additionally, in this example, the quantum-well layer is made from GaN,and the width of the well is selected at 7 nm, 5 nm and 3 nm. Further,in the case of the strained quantum-well layer, the barrier layer ismade from AlGaN (Al mol ratio: 40%). The effective mass of positiveholes on the ordinate are relative values standardized by a bulk valuewithin a plane perpendicular to the c-axis. The other quantum-wellstructures are also provided so as to exhibit specific characteristics.

On the basis of the above examination, the present inventors have foundthat the effect obtained when the growth plane of the above lightemitting region, particularly, active layer is inclined 90° from the(0001) plane and the strained amount is specified as e_(c)=e_(a) is alsoproduced when the inclination angle is reduced to 65° and the range ofthe strained amount is specified as e_(c)≦1.1×e_(a).

As a result, it has been found that the inclination angle is desirableto be set for each of the following two cases.

(1) In the case where the strained amount is specified ase_(c)≦1.1×e_(a), the inclination angle is set in a range of 70° to 90°.

(2) In the case where the strained amount is specified ase_(c)>1.1×e_(a), the inclination angle is set in a range of 3° to 70°,preferably, 5° to 70°, more preferably, 10° to 70°.

And, on the basis of this knowledge as well as the knowledge associatedwith the isotropy of energy, it has been found that the above effect,that is, the reduction in threshold carrier density due to the reductionin effective mass of carriers can be achieved even if the growthdirection is inclined in any plane orientation insofar as theinclination angle is set in the above-described angle range.

Further, according to the configuration of the present invention, theoptical gain of the quantum-well active layer exhibits a large opticalanisotropy. In the case of the strained amount specified as e_(c)=e_(a),in the inclination angle range of 70° to 90°, since the band state atthe top of the valence band becomes only the P-orbit component parallelto the (0001) plane, the transition probability can be made twice thatin the conventional quantum-well in which the oriented plane is taken asthe (0001) plane by taking the end surface of the laser resonator as the(0001) plane or a plane equivalent thereto. In this case, since thegallium nitride based semiconductor crystal having the wurtzite typestructure exhibits good cleavage along the (0001) plane, the laserresonator structure can be easily obtained by a cleavage method like theconventional semiconductor laser made from the zinc-blend type crystal.On the other hand, in the case of the strained amount specified ase_(c)>1.1xe_(a), in the inclination angle range of 10° to 70°, since theband state at the top of the valence band becomes only the P-orbitcomponent parallel to the (−1,2,−1,0) plane, the optical transitionprobability can be made twice that in the conventional quantum-well inwhich the oriented plane is taken as the (0001) plane by taking the endsurface of the laser resonator as the (−1,2,−1,0) plane or a planeequivalent thereto.

A relationship between the crystal growth axial direction and the endsurface of an optical resonator in a semiconductor laser device will bebriefly described. In the case where the oriented plane is taken as the(0001) plane, the state of positive holes becomes a hybrid orbit of Pxand Py, and accordingly, the polarization due to laser light cannot beefficiently formed For example, if the polarization direction of laserlight is taken as the X-direction, the Py component does not contributeto the polarization. On the contrary, at a plane inclined from the(0001) plane, the P-orbit can be fixed in one axial direction.Accordingly, by selecting the polarization direction along such adirection, all of the positive holes can contribute to the polarization.In this case, the cleavage plane is required to be set in parallel tothe polarization direction.

As described above, the reduction in threshold current value of asemiconductor light emitting device results from the reduction ineffective mass of positive holes of a semiconductor material.Hereinafter, there will be supplementarily described the conception ofsuch a phenomenon for a clearer understanding thereof.

As is apparent from the theory of solid-state physics, the reduction ineffective mass of positive holes means that the state density of thevalence band becomes small. In the case where a semiconductor laserdevice is configured using semiconductor stacked layers in such a state,it satisfies the laser oscillation condition at a small carrier density.This is an effect obtained mainly by a shear component of strain (shearstrain).

The reason why the inclination of the crystal growth axis of aquantum-well semiconductor stacked layer structure leads to thereduction in effective mass of positive holes may be considered asfollows. In the wurtzite type semiconductor crystal grown in the c-axisdirection, the effective mass of positive holes is generally very largeirrespective of the bulk state or quantum-well structure. This is due tothe fact that the P-orbit component constituting the valence band issymmetric within a plane perpendicular to the c-axis (that is, withinthe x-y plane). FIG. 9 is a diagram of a band structure of the (0001)plane showing such a symmetric state. Referring to FIG. 9, since theX-state band is overlapped to the Y-state band, the effective mass ofpositive holes becomes large.

On the contrary, for the band structure of a plane other than the (0001)plane, the symmetry of each of the X-state band and Y-state band islost. Since the X-state band and the Y-state band are separated fromeach other on the energy axis, the effective mass of positive holes isreduced. To be more specific, for the (0001) plane, the positive holestake two states in the Px and Py directions; however, for a planeinclined from the (0001) plane, the positive holes take only state inone direction. This means that the state density is reduced. In the caseof the strained quantum-well, at a plane inclined from the (0001) plane,the symmetry is largely lost because shear strain is caused as describedabove. FIG. 10 shows an example of the band structure of a plane otherthan the (0001) plane.

As described above, the reduction in effective mass of carriers leads tothe reduction in state density of carriers, this will be briefly,qualitatively described below. In general, the state density representsthe number of states per unit volume and unit energy. In the case of theeffective mass being large, the range of momentum per unit energy iswide. That is to say, since the effective mass of carriers is large,more roughly, since the carriers are heavy, the unit energy is notincreased unless the momentum is increased. The wide range of momentummeans that the carriers take various states of momentum. On thecontrary, the reduction of the effective mass of the carriers makesnarrower the range of momentum, that is, makes relatively smaller thepossible states of momentum of the carriers.

The reason why the threshold current required for light emission isreduced by using a semiconductor material having such characteristicswill be briefly described with reference to FIGS. 11A and 11B. Ingeneral, for a semiconductor material, the state density of electrons(conduction band) is smaller than the state density of positive holes(valence band). Accordingly, electrons are concentrated at the band end,that is, in a small momentum state, while positive holes are distributedeven in a large momentum state because the state density of the positiveholes is large as described above. Light emission occurs by emissioncombination of electrons and positive holes, and in this case, as isapparent from the theory of solid-state physics, the momentum isretained. Accordingly, the positive holes in the large momentum statebecome useless, to thereby increase the threshold current value requiredfor light emission. FIG. 11A shows such a state.

On the other hand, by reducing the state density of the valence band tomake it closer to that of the conduction band, the above useless stateof the positive holes can be reduced. This is effective to increase theoptical transition probability contributing to emission combination ofelectrons and positive holes, and hence to reduce the threshold currentrequired for light emission. FIG. 11B shows such a state.

2-3: Second Type Semiconductor Light Emitting Device

The second type semiconductor light emitting device of the presentinvention will be described below. The definition of a light emittingregion which may be configured as a quantum-well structure in thefollowing description is the same as that described in the first typesemiconductor light emitting device of the present invention.

As a first aspect of the second type semiconductor light emitting deviceof the present invention, there is provided a semiconductor lightemitting device composed of at least compound semiconductors, the deviceincluding at least a first conduction type cladding layer; a secondconduction type cladding layer; and an active layer region having aquantum-well structure, which region is put between the cladding layersand which has a well layer and barrier layers each having a forbiddenband width larger than that of the well layer; characterized in that theoriented plane of the quantum-well active region is a plane inclined 70°to 90° from the (0001) plane or a plane equivalent thereto. In thiscase, as described above, the inclination angle may have a deviation,that is, a tolerance of 5° or less. Further, as described above, thetolerance is applied to inclination in the directions other than theinclination direction from the (0001) plane.

As one example for carrying out the above configuration, a semiconductorlayer constituting the light emitting device is sometimes epitaxiallygrown on (upper portion of) a misoriented substrate. In general, the(0001) plane of a sapphire substrate having a hexagonal crystalstructure is taken as a principal plane for expitaxial growth; however,in the case of adopting the misoriented substrate, a plane inclined aspecific angle from the (0001) plane is taken as the principal plane forexpitaxial growth The epitaxial growth using the misoriented substrateis good in quality of the grown crystal (crystallinity) and doping of animpurity upon growth of crystal, and in the case where the epitaxialgrowth using the oriented substrate is adopted for the presentinvention, the above inclination angle range is further allowed to havea tolerance of 5°. The tolerance of 5° can be applied to the case usingno oriented substrate depending on the degree of reduction in requiredoperational current, in consideration of a relationship between theangle of the expitaxial growth plane of an active layer and thethreshold carrier density.

2-4: Third Type Semiconductor Light Emitting Device

The third type semiconductor light emitting device of the presentinvention will be described below. The definition of a light emittingregion which may be configured as a quantum-well structure in thefollowing description is the same as that described in the first typesemiconductor light emitting device of the present invention.

As a first aspect of the third type semiconductor light emitting deviceof the present invention, there is provided a semiconductor lightemitting device including at least a quantum-well active layer regionmade from a semiconductor material of a hexagonal system, characterizedin that the crystal growth direction of the quantum-well active layerregion is parallel to an axis which is inclined 3° to 70° from the[0001] axis (c-axis) or an axis equivalent thereto. As the hexagonalcrystal, there may be preferably used a wurtzite type crystal.

The inclination angle may be in a range of 5° or more. Further, theinclination angle is preferably in a range of 5° to 70°, morepreferably, 10° to 60°, and most preferably, 20° to 55°. The inclinationangle will be more clearly apparent from the description of FIG. 8.

In the case of a semiconductor laser device adopting a strainedquantum-well structure in which the crystal growth axis is specified inparallel to an axis inclined 20° to 55° from the [0001] axis (c-axis),the end surface of the laser resonator is preferably taken as the (−1,2,−1,0) plane or a plane equivalent thereto.

Hereinafter, the selection of the crystal growth orientation of aquantum-well layer and the adoption of a reflection plane of a resonatorin the case of a semiconductor laser device will be more concretelydescribed.

Example of Selection of Crystal Growth Orientation of Quantum-well Layer

(1) The semiconductor laser device includes at least a strainedquantum-well active layer region made from a wurtzite type semiconductormaterial, wherein the crystal growth plane is taken as a plane which isinclined 3° or less or 5° or less from the (1,0,−1,N) plane (N=1, 2, or3) or a plane equivalent thereto.

(2) The semiconductor laser device includes at least a strainedquantum-well active layer region made from a wurtzite type semiconductormaterial, wherein the crystal growth plane is taken as a plane which isinclined 5° or less from the (−1,2,−1, N) plane (N=3, 4, or 5) or aplane equivalent thereto.

Hereinafter, the selection of the crystal growth orientation of aquantum-well layer and the adoption of a reflection plane of a resonatorin the case of a semiconductor laser device will be more concretelydescribed.

Example of Adoption of Reflection Plane of Resonator of SemiconductorLaser Device

(1) A strained quantum-well active layer region constitutes part of thesemiconductor laser resonator, and the end surface of the semiconductorlaser resonator is parallel to a plane containing both a crystal growthaxis in the strained quantum-well active layer region and the [0001]axis (c-axis).

(2) A quantum-well active layer region or strained quantum-well activelayer region constitutes part of the semiconductor laser resonator, andthe end surface of the semiconductor laser resonator is parallel to aplane containing both a crystal growth axis in the quantum-well activelayer region or strained quantum-well active layer region and the [0001]axis (c-axis), and is equivalent to the (−1,2,−1.0) plane.

(3) A quantum-well active layer region or strained quantum-well activelayer region constitutes part of the semiconductor laser resonator, andthe end surface of the semiconductor laser resonator is parallel to aplane containing both a crystal growth axis in the quantum-well activelayer region or strained quantum-well active layer region and the [0001]axis (c-axis), and is equivalent to the (1,0,−1,0) plane.

2-5: Other Supplement Points of Semiconductor Light Emitting Device ofthe Present Invention

In the case where the configuration of the above-described semiconductorlight emitting device is applied to a surface emission type lightemitting device, it is needless to say that a semiconductor laser devicegood in polarization characteristic can be obtained by theabove-described effect.

In a typical example of each of the second type and third typesemiconductor light emitting devices of the present invention, thecrystal growth plane of a gallium nitride based semiconductor isinclined from the (0001) plane, and accordingly, it may be consideredthat the crystal plane of a principal substrate is required to bematched with the crystal growth plane. However, there have been reportedexperimental data in which the GaN crystal with the (1,−1,0,0) planetaken as the growth plane is grown on the (0001) principal plane of aSiC substrate and the GaN crystal with the (1,1,−2,0) plane taken as thegrowth plane is grown on the (1,−1,0,2) principal plane of a sapphiresubstrate. Accordingly, the crystal growth plane of a gallium nitridebased semiconductor is not required to be matched with the plane indexof the principal crystal plane of a substrate.

While the structure of each of the second type and third typesemiconductor light emitting devices of the present invention has beendescribed with reference to the example using the light emitting regionof the quantum-well structure, such a structure is not limited to thelight emitting region of the quantum-well structure. The presentinvention can be applied, for example, to the case where the lightemitting region is configured as a single active layer joined to opticalguide layers or cladding layers.

A gallium nitride based semiconductor crystal applied to each of thefirst type, second type and third type semiconductor light emittingdevices of the present invention will be additionally described below.The semiconductor material is the group Ill-V compound semiconductorwhich necessarily contains nitrogen (N). In the above description, thegroup Ill-V compound semiconductor is called the gallium nitride basedsemiconductor for convenience; however, it does not necessarily containgallium (Ga) as the group III element but may contain at least one kindselected from the group III elements such as aluminum (Al), gallium andindium (In). Further, with respect to the group V elements, the groupIII-V compound semiconductor may contain not only nitrogen (N) but alsoother group V elements such as phosphorous (P), arsenic (As) andantimony (Sb). That is to say, the compound semiconductor of the presentinvention represented by a gallium nitride based semiconductor may becalled a III-V compound semiconductor containing at least nitrogen ornitride semiconductor.

As the semiconductor material forming the quantum-well structure, theremay be used a wurtzite type semiconductor material represented by agallium nitride based semiconductor which is expressed by a generalstructural formula of In_(x)Al_(y)Ga_(1−x−y)N_(1−a−b)As_(a)P_(b) where0≦x≦1, 0≦y≦1, 0≦a<1, 0≦b<1, x+y≦1, and a+b<1.

The active layer is preferably made from CaN, InGaN, InGaAlN, GaNP,GaNAs, InGaNP, InGaNAs, GaAlNP or GaAlNAs. The well layer is preferablymade from GaN or InGaN. The clad is typically made from GaAlN, AlN, GaNor InGaAlN.

The thickness of each of the well layer or barrier layer for thequantum-well semiconductor stacked structure may be the same as that ofthe conventional one. In general, the thickness of the well layer is ina range of 2 nm to 15 nm, and the thickness of barrier layer is in arange of 3 nm to 15 nm. Preferably, the thickness of the well layer isin a range of 2 nm to 8 nm, and the thickness of the barrier layer is ina range of 4 nm to 8 nm.

In the preferable quantum-well active layer composed of the well layerand barrier layers, the lattice constant of the well layer is largerthan that of the barrier layer, and a compressive strain is applied tothe well layer. The lattice constant of the strained well layer isdesirable to be larger than that of the unstrained well layer by 0.6% to2.0%.

The substrate for formation of such a semiconductor stacked body may bemade from a material on which the wrutzite type crystal can be grown,for example, sapphire, GaN, spinel, SiC, ZnO, MgO, MnO, SiO₂, or AlN. Inparticular, the sapphire substrate or GaN substrate is preferably used.

In manufacture of a semiconductor light emitting device, the known meansmay be used insofar as the above-described configuration of the presentinvention is satisfied. For example, a buffer layer for improving thecrystallinity may be provided between a substrate for crystal growth anda quantum-well semiconductor stacked layer structure in accordance withthe known method. Also, the withdrawal of an electrode from aninsulating sapphire substrate, provision of a so-called contact layerfor forming another electrode withdrawn from an upper portion of thesemiconductor stacked layer structure, or formation of a layer insertedoptionally for improving the crystallinity may be performed inaccordance with the known method. It should be understood that suchadditional configurations and modified configurations fall within thescope of the present invention.

In addition, such a semiconductor stacked layer structure can be ofcourse used, in addition to a semiconductor light emitting device, othersemiconductor devices required for reducing the effective mass ofpositive holes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the dependence of the density of amodulation-doped impurity on the maximum gain in a GaN/AlGaNquantum-well;

FIG. 2 is a diagram showing the dependence of the density of amodulation-doped impurity on a threshold current density in a GaN/AlGaNquantum-well;

FIG. 3 is a diagram showing the structure of a gallium nitride basedsemiconductor crystal;

FIG. 4 is a diagram showing the growth plane of a gallium nitride basedsemiconductor crystal according to a second device configuration of thepresent invention;

FIG. 5 is a diagram illustrating plane orientation indices of ahexagonal lattice;

FIGS. 6A, 6B, 6C and 6D are diagrams each showing energy dispersion inthe upper portion of a valence band in a GaN/AlGaN quantum-well with theoriented plane taken as the (1,0,−1,0) plane;

FIG. 7 is a diagram showing a relationship among a crystal growth plane,lattice strain, and optical gain in a GaN/AlGaN quantum-well activeregion;

FIG. 8 is a diagram showing the results of theoretically calculating arelationship between the effective mass of positive holes and aninclination angle from the (0001) plane;

FIG. 9 is a diagram showing a band structure of a semiconductormaterial;

FIG. 10 is a diagram showing another band structure of a semiconductormaterial;

FIGS. 11A and 11B are band structure diagrams illustrating the opticaltransition probability;

FIG. 12 is a schematic diagram illustrating a photomagnetic recordingapparatus;

FIG. 13 is a diagram showing a relationship between an operationalcurrent and an optical output;

FIG. 14 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 15 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 16 is a schematic view illustrating an optical recording apparatus;

FIG. 17 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 18 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 19 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 20 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 21 is a sectional view of a semiconductor laser device according toone embodiment of the present invention;

FIG. 22 is a perspective view of a semiconductor laser device accordingto one embodiment of the present invention; and

FIGS. 23A, 23B, 23C and 23D are sectional views showing a semiconductorlaser device according to one embodiment of the present invention in theorder of manufacturing steps thereof.

BEST MODE FOR CARRYING OUT THE INVENTION

The mode for carrying out the present invention will be described indetail by way of embodiments with reference to the drawings.

<First Embodiment>

Description will be made of optical information processing equipmenttypically configured as a photomagnetic disk apparatus.

First, the general configuration of a photomagnetic disk apparatus willbe schematically described. FIG. 12 is a schematic configuration viewshowing one example of a photomagnetic disk apparatus. A photomagneticdisk apparatus 100 is basically composed of an optical head 101, acontrol circuit, and a mechanical system including, for example, amotor.

First, in the optical head 101, linearly polarized light emitted from asemiconductor laser device 102 passes through a light branching element103 and is focused on an photomagnetic disk 105 through an objectivelens 104. Upon reproducing of a signal, the polarization direction ofthe light reflected from the photomagnetic disk 105 is turned about 1°reversely depending on whether or not a light spot is located on arecording domain, to carry recording information. Upon recording of asignal, a magnetic field applying coil 106 applies to the photomagneticdisk 105 a magnetic field in the direction reversed to the previouslyinitialized magnetization direction of the photomagnetic disk 105, toreverse the magnetization of only a region whose temperature has beenincreased to a temperature higher than a transition temperature (Curiepoint) of a magnetic recording film by irradiation of light spot,thereby forming a magnetic domain.

The light reflected from the photomagnetic disk 105 passes through theobjective lens 104 again, being reflected from the light branchingelement 103, and enters a second light branching element 107. First, thelight reflected from the second light branching element 107 enters aservo-signal detecting photo detector 108. At the photo detector 108, aplurality of signals necessary for computing an out-of-focus signal anda tracking error signal are detected from the light by split photodetectors contained in the photo detector 108. On the other hand, thelight having passed through the second light branching element 107 isseparated, by a polarized light separating element 109, into linearlypolarized light components inclined +45° and −45° from the polarizationdirection of the linearly polarized light having entered thephotomagnetic disk 106, which light components enter two split photodetectors 110. Accordingly, in the case of no-turn of the polarizedlight by the photomagnetic disk, the quantities of the two linearlypolarized light components are equal to each other; and in the case ofturn of the polarized light, the above two light quantities areincreased/decreased depending on the turning direction thereof. That isto say, a difference between outputs from the two split photo detectorsbecomes a photomagnetic reproducing signal.

The electric input or output into or from the optical head 101 isperformed via flexible substrates 111 and 112. In this embodiment, thedrive of the semiconductor laser device 102 and the input or output intoor from the servo-signal detector 108 are performed via the flexiblesubstrate 111, and the input or output into or from the two split photodetectors 110 is performed via the flexible substrate 112.

The semiconductor laser 102 controls the flickering of the light by alaser drive circuit 116 on the basis of a recording waveform. Therecording waveform is created by a recording waveform creating circuit115 in accordance with digital information supplied from a buffer memory114 for storing user data 113 and is supplied to the laser drive circuit116 therefrom.

The current output from the servo-signal detecting photo detector 108 issubjected to current-voltage conversion and amplification by aservo-signal computing circuit 117, to create an out-of-focus signal, atracking error signal, and a head position control signal. The above twoerror signals are fed back to an objective lens actuator 118. Theobjective lens actuator 118 performs closed loop control on the basis ofthe error signals such that the focused light spot is usually located atan information track on the recording film plane of the photomagneticdisk 1105. On the other hand, the head position control signal isinputted in a head moving mechanism 119 in order that the optical head101 is located substantially in the vicinity of a reproducing track. Fora photomagnetic disk for recording digital information, the head movingmechanism 119 generally performs closed loop control; however, for a CD,DVD or MD, it often performs opened loop control.

The two split photo detectors 110, each of which is generally configuredto contain an amplifier, output voltage signals into a signal detectingcircuit 120 via the flexible substrate 112, being processed in terms ofdifferential amplification, equalization, conversion into binary form,and decoding, to thus reproduce digital information. The outputinformation is stored in the buffer memory 114.

The photomagnetic disk 105 is rotated by a spindle motor 121. Therotation of the photomagnetic disk 105 is controlled by a spindle motordrive circuit 122. The magnetization direction of the photomagnetic disk105 is controlled by a magnetic field applying coil control circuit 123.In addition, the above control circuits are all controlled by acontroller 124.

In this embodiment, as the semiconductor laser device 102, there wasprepared the following one (#1, #2, or #3). Either of the semiconductorlaser devices (#1, #2, and #3) is allowed to sufficiently achieve theobject of the present invention.

Semiconductor semiconductor semiconductor laser device laser devicelaser device (#1) (#2) (#3) wavelength (nm) 350-550 350-550 350-550threshold current (mA) 8   11   6   threshold current 1.0 1.2 0.8density (kA/cm²) slope efficiency 0.3-0.4 0.4-0.5 0.4 temperature 120 K130 K 120 K characteristic oscillation delay time 1 ns or less 2 ns 1 nsor less polarization TE mode TE mode TE mode

Each of the above light sources is capable of realizing: emission oflight having a wavelength in a range of 350 nm to 550 nm; a thresholdcurrent density of the light source in a range of 2.2 kA/cm² or less;and a recording density of the above recording medium in a range of 15GB or more.

To achieve the object of the present invention, the threshold currentdensity of the above light source is preferably in a range of 1.0 kA/cm²or less, more preferably, in a range of 0.8 kA/cm² or less, and furtherthe threshold current is preferably in a range of 10 mA or less.

FIG. 13 is a diagram showing a relationship between an operationalcurrent and an optical output as a result of using, in this embodiment,a semiconductor laser device of the present invention and,alternatively, a conventional semiconductor laser device. From thisdiagram, it will be understood that the semiconductor laser device ofthe present invention is superior to the conventional one.

Hereinafter, configuration examples of semiconductor laser devicesapplicable to this embodiment will be described.

FIRST EXAMPLE

FIG. 14 is a structural sectional view, seen from a plane crossing anoptical axis, showing a semiconductor laser device as a first example ofthe present invention.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on a sapphire substrate1 with the (0001) plane taken as the principle plane thereof, a lowtemperature buffer layer 2 made from n-type GaN, a high temperaturebuffer layer 3 made from n-type GaN, an n-type Al_(0.2)Ga_(0.8)Ncladding layer 4, an n-type GaN optical guide layer 5, a GaInNmulti-quantum well active region 6, a p-type GaN optical guide layer 7,a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaN cap layer9. Each of these layers is formed by epitaxial growth by the usual metalorganic vapor phase epitaxial growth method. The thickness of each layeris set at 0.05 μm (low temperature buffer layer 2), 3 μm (hightemperature buffer layer 3), 0.3 μm (n-type cladding layer 4), 0.1 μm(n-type optical guide layer 5), 0.1 μm (p-type optical guide layer 7),0.3 μm (p-type cladding layer 8), and 0.5 μm (cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.85)In_(0.15)N well layers 21 each having a thickness of 2 nm andthree pieces of n-doped Ga_(0.95)In_(0.05)N barrier layers 22 eachhaving a thickness of 2 nm are alternately stacked to each other.

Here, the density of the impurity doped in the barrier layer 22 is setsuch that the density of electrons released from the impurity andlocally present in the well layer becomes 3×10¹⁹ cm⁻³. A difference ΔEgin band gap energy between the cladding layer and quantum-well layer isset at 0.8 ev. Next, the stacked structure on the substrate 1 isannealed at a temperature ranging from 400 to 800° C. for 20 to 60 minfor activating the p-type layers. Then, as shown in FIG. 14, parts ofthe each grown semiconductor layers are subjected to dry-etching, toform a ridge stripe having a width of 2 μm. The ridge stripe structureis useful for current constriction and more useful for reduction inthreshold value. To form an n-type electrode, the n-type GaN bufferlayer 3 is exposed. Subsequently, a metal film made from Au or Al isformed, followed by patterning, to form both a p-side electrode 10 andan n-side electrode 11. Then, the growth layers are subjected todry-etching from the upper side, so that the resonance plane is formedinto a resonator length of 400 μm. In general, a plurality ofsemiconductor laser devices are formed on one wafer, and accordingly,the stacked structure on the substrate 1 is separated into respectivelaser devices by dicing. In the semiconductor laser device thusobtained, continuous oscillation was realized at a threshold current ofabout 8 mA at room temperature and the oscillation wavelength was about405 nm.

SECOND EXAMPLE

A semiconductor laser device in this example of the present inventionwill be described with reference, also, to FIG. 14.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on a sapphire substrate1 with the (1,0,−1,2) plane taken as the principle plane thereof, a lowtemperature buffer layer 2 made from n-type GaN, a high temperaturebuffer layer 3 made from n-type GaN, an n-type Al_(0.2)Ga_(0.8)Ncladding layer 4, an n-type GaN optical guide layer 5, a GaInNmulti-quantum well active region 6, a p-type GaN optical guide layer 7,a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaN cap layer9. Each of these layers is formed by epitaxial growth by the usual metalorganic vapor phase epitaxial growth method. The thickness of each layeris set at 0.05 μm (low temperature buffer layer 2), 3 μm (hightemperature buffer layer 3), 0.3 μm (n-type cladding layer 4), 0.1 μm(n-type optical guide layer 5), 0.1 μm (p-type optical guide layer 7),0.3 μm (p-type cladding layer 8), and 0.5 μm (cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.85)In_(0.15)N well layers 21 each having a thickness of 2 nm andthree pieces of undoped GaN barrier layers 22 each having a thickness of4 nm are alternately stacked to each other. The oriented plane of thequantum-well is the (1,0,−1,0) plane. Here, the composition of the welllayer 21 is set such that the displacement of the lattice constant fromthe lattice constant in the unstrained state becomes e_(a)=0.85%,e_(c)=0.9%, and a biaxial compressive strain is applied.

Next, the stacked structure on the substrate 1 is annealed at atemperature ranging from 400 to 800° C. for 20 to 60 min for activatingthe p-type layers. Then, as shown in FIG. 14, parts of the each grownsemiconductor layers are subjected to dry-etching, to form a ridgestripe having a width of 2 μm To form an n-type electrode, the n-typeGaN buffer layer 3 is exposed. Subsequently, a metal film made from Auor Al is formed, followed by patterning, to form both a p-side electrode10 and an n-side electrode 11. In general, a plurality of semiconductorlaser devices are formed on one wafer, and accordingly, the stackedstructure on the substrate 1 is cleaved into resonators each having alength of 400 μm along the (0001) plane, followed by formation of acoating on end surfaces thereof for improving reflection index, tomanufacture semiconductor laser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 11 mA at room temperatureand the oscillation wavelength was about 405 nm.

THIRD EXAMPLE

FIG. 15 is a structural sectional view, taken from a plane crossing anoptical axis, showing a semiconductor laser device as a third example ofthe present invention.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on an n-type SiCsubstrate 51 with the (1,0,−1,0) plane taken as the principle planethereof, a low temperature buffer layer 2 made from n-type GaN, a hightemperature buffer layer 3 made from n-type GaN, an n-typeAl_(0.2)Ga_(0.8)N cladding layer 4, an n-type GaN optical guide layer 5,a GaInN multi-quantum well active region 6, a p-type GaN optical guidelayer 7, a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaNcap layer 9. Each of these layers (and region) is formed by epitaxialgrowth by the usual metal organic vapor phase epitaxial growth method.The thickness of each layer is set at 0.05 μm (low temperature bufferlayer 2), 3 μm (high temperature buffer layer 3), 0.3 μm (n-typecladding layer 4), 0.1 μm (n-type optical guide layer 5), 0.1 μm (p-typeoptical guide layer 7), 0.3 μm (p-type cladding layer 8), and 0.5 μm(cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.85)In_(0.15)N well layers 21 each having a thickness of 2 nm andthree pieces of n-doped Ga_(0.95)In_(0.05)N barrier layers 22 eachhaving a thickness of 4 nm are alternately stacked to each other.

Here, the density of the impurity doped in the barrier layer 22 is setsuch that the density of electrons released from the impurity andlocally present in the well layer becomes 4.0×10¹⁹ cm⁻³. The orientedplane of the quantum-well is the (1,0,−1,0) plane. Here, the compositionof the well layer 21 is set such that the displacement of the latticeconstant from the lattice constant in the unstrained state becomese_(a=)0.85%, e_(c=)0.9%, and a biaxial compressive strain is applied.The biaxial strain means a strain caused by a stress applied to crystallattices of different kinds of crystal layers resulting from adifference between both crystal lattices at an interface at which thedifferent kinds of crystal layers are bonded to each other, which strainmay be simply called “strain” or “lattice strain”. A difference ΔEg inband gap energy between the cladding layer and quantum-well layer is setat 0.8 eV. Next, the stacked structure on the substrate 1 is annealed ata temperature ranging from 400 to 800° C. for 20 to 60 mm for activatingthe p-type layers.

Then, part of the growth layers are subjected to dry-etching, to form aridge stripe having a width of 2 μm.

Subsequently, a metal film made from such as Au or Al is formed,followed by patterning, to form both a p-side electrode 10 and an n-sideelectrode 11. The stacked structure on the substrate 1 is then cleavedinto resonators each having a length of 400 μm along the (0001) plane,followed by formation of a coating on end surfaces thereof for improvingreflection index, to manufacture semiconductor laser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 6 mA at room temperatureand the oscillation wavelength was about 410 nm.

<Second Embodiment>

One example of optical information processing equipment of a type makinguse of a state transition such as a phase transition of a recordingmedium will be described.

FIG. 16 is a basic configuration view showing one example of an opticaldisk apparatus. Reference numeral 211 designates a disk on which anoptical recording medium for optical recording is provided; 212 is amotor for rotating the disk; 213 is an optical pickup; and 217 is acontrol unit for controlling these components. The optical pickup 213has a lens system 214, a light source 215 such as a semiconductor laserdevice, and a photo detector 216.

The general configurations of such an optical disk apparatus have beenvariously reported, and therefore, only an important configuration willbe briefly described. The optical disk apparatus is basically classifiedinto a read-only type, a write once/read many times type, and rewritabletype depending on the kind of a recording material.

The reproducing of information is performed by optically reading changesin light reflected from micro-holes (state change portions of recordingmedium) recorded in the disk 211 by the photo detector 216. In addition,the optical disk apparatus can be operated using the usual opticalrecording medium.

In the case of the read-only type, information is previously recorded ina recording medium of a read-only type, which medium is typically madefrom aluminum or a plastic material.

In recording information, the state of a recording material is thermallychanged by modulating laser light, to perform recording in a row. Therecording is performed with the disk rotated (moved) by the motor. Assuch a light source, the light source of the present invention can beused.

As the light source of such an optical disk apparatus, the semiconductorlaser device shown in each of the examples in the first embodiment canbe used, and also a semiconductor laser device to be manufactured inaccordance with the following embodiments 3 to 9 can be used.

At present, the wavelength of a semiconductor laser device used for anoptical disk apparatus is generally in a range of 630 nm or more. Ingeneral, the recording density of an optical disk is proportional to thesquare of the inverse of the wavelength of light emitted from the lightsource. Accordingly, the practical use of a blue-color based,blue-violet color based, or violet color based semiconductor laserdevice having an oscillation wavelength typically in a range of 430 nmto 550 nm according to the present invention allows realization ofhigher density recording of the optical disk apparatus. Further, thereduction in threshold current of the semiconductor laser deviceaccording to the present invention contributes to the prolonged servicelife of the optical disk apparatus, and therefore, it is extremelyadvantageous from the viewpoint of practical use.

Each of the above light sources is capable of realizing: emission oflight having a wavelength in a range of 350 nm to 550 nm; a thresholdcurrent density of the light source in a range of 2.2 kA/cm² or less;and a recording density of the above recording medium in a range of 15GB or more.

To achieve the object of the present invention, the threshold currentdensity of the above light source is preferably in a range of 1.0 kA/cm²or less, more preferably, in a range of 0.8 kA/cm² or less, and furtherthe threshold current is preferably in a range of 10 mA or less.

In this way, the optical information equipment of the present inventionis capable of realizing higher recording density and higher reliability.

Hereinafter, a configuration example of a semiconductor laser deviceapplicable to this embodiment will be described.

FOURTH EXAMPLE

A semiconductor laser device in this embodiment will be described withreference to FIG. 14.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on a sapphire substrate1 with the (1,0,−1,2) plane taken as the principle plane thereof, a lowtemperature buffer layer 2 made from n-type AlN, a high temperaturebuffer layer 3 made from n-type AlN, an n-type Al_(0.2)Ga_(0.8)Ncladding layer 4, an n-type GaN optical guide layer 5, a GaInNmulti-quantum well active region 6, a p-type GaN optical guide layer 7,a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaN cap layer9. Each of these layers is formed by epitaxial growth by the usual metalorganic vapor phase epitaxial growth method. The thickness of each layeris set at 0.05 μm (low temperature buffer layer 2), 3 μm (hightemperature buffer layer 3 ), 0.3 μm (n-type cladding layer 4 ), 0.1 μm(n-type optical guide layer 5 ), 0.1 μm (p-type optical guide layer 7 ),0.3 μm (p-type cladding layer 8 ), and 0.5 μm (cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.85)In_(0.15)N well layers 21 each having a thickness of 2 nm andthree pieces of undoped GaN barrier layers 22 each having a thickness of4 nm are alternately stacked to each other. The oriented plane of thequantum-well is the (1,0−1,2) plane. Here, the composition of the welllayer 21 is set such that the displacement of the lattice constant fromthe lattice constant in the unstrained state becomes e_(a)=0.85%,e_(c)=1,4%, and a biaxial compressive strain is applied. Next, thestacked structure on the substrate 1 is annealed at a temperatureranging from 400 to 800° C. for 20 to 60 min for activating the p-typelayers.

Then, part of the grown semiconductor layers are subjected todry-etching to form a ridge stripe having a width of 2 μm. To form ann-type electrode, the n-type GaN buffer layer 3 is exposed.Subsequently, a metal film made from Au or Al is formed, followed bypatterning, to form both a p-side electrode 10 and an n-side electrode11. The stacked structure on the substrate 1 is cleaved into resonatorseach having a length of 400 μm along the (1,2,−1,0) plane, followed byformation of a high reflection coating, to manufacture semiconductorlaser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 10 mA at room temperatureand the oscillation wavelength was about 412 nm.

<Third Embodiment>

FIG. 17 is a structural sectional view, taken from a plane crossing anoptical axis, showing a semiconductor laser device in this embodiment.

The semiconductor laser device having a multi-quantum well structure inthis embodiment is formed by sequentially stacking, on a sapphiresubstrate 1 with the (0001) plane taken as the principle plane thereof,a low temperature buffer layer 2 made from n-type GaN, a hightemperature buffer layer 3 made from n-type GaN, an n-typeAl_(0.2)Ga_(0.8)N cladding layer 4, an n-type GaN optical guide layer 5,a GaInN multi-quantum well active region 6, a p-type GaN optical guidelayer 7, a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaNcap layer 9. Each of these layers is formed by epitaxial growth by theusual metal organic vapor phase epitaxial growth method. The thicknessof each layer is set at 0.05 μm (low temperature buffer layer 2), 3 μm(high temperature buffer layer 3), 0.3 μm (n-type cladding layer 4), 0.1μm (n-type optical guide layer 5), 0.1 μm (p-type optical guide layer7), 0.3 μm (p-type cladding layer 8), and 0.5 μm (cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.8)In_(0.2)N well layers 21 each having a thickness of 3 nm andthree pieces of n-doped Ga_(0.95)In_(0.05)N barrier layers 22 each beingdoped with an n-type impurity at the density of 2×10¹⁹ cm⁻³ and having athickness of 5 nm are alternately stacked to each other.

Here, the density of the impurity doped in the barrier layer 22 is setsuch that the density of electrons released from the impurity andlocally present in the well layer becomes 4.5×10¹⁹ cm⁻³. A differenceΔEg in band gap energy between the cladding layer and quantum-well layeris set at 0.8 eV.

Next, the stacked structure on the substrate 1 is annealed at atemperature ranging from 400 to 800° C. for 20 to 60 min for activatingthe p-type layers. Then, to form an n-type electrode, as shown in FIG.15, part of the grown semiconductor layers are subjected to dry-etching,to expose the n-type GaN buffer layer 3. Next, a metal film made fromsuch as Au or Al is formed, followed by patterning, to form both ap-side electrode 10 and an n-side electrode 11. In general, a pluralityof semiconductor laser devices are formed on one wafer, and accordingly,the stacked structure on the substrate 1 is separated into respectivelaser devices by dicing.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 50 mA at room temperatureand the oscillation wavelength was about 410 nm.

<Fourth Embodiment>

FIG. 18 is a structural sectional view, taken from a plane crossing anoptical axis, showing a semiconductor laser device in this embodiment.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on a sapphire substrate31 with the (1,0,−1,2) plane taken as the principle plane thereof, a lowtemperature buffer layer 2 made from n-type GaN, a high temperaturebuffer layer 3 made from n-type GaN, an n-type Al_(0.2)Ga_(0.8)Ncladding layer 4, an n-type GaN optical guide layer 5, a GaInNmulti-quantum well active region 6, a p-type GaN optical guide layer 7,a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaN cap layer9. Each of these layers is formed by epitaxial growth by the usual metalorganic vapor phase epitaxial growth method. The thickness of each layeris set at 0.05 μm (low temperature buffer layer 2), 3 μm (hightemperature buffer layer 3), 0.3 μm (n-type cladding layer 4), 0.1 μm(n-type optical guide layer 5), 0.1 μm (p-type optical guide layer 7),0.3 μm (p-type cladding layer 8), and 0.5 μm (cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.8)In_(0.2)N well layers 41 each having a thickness of 4 nm andthree pieces of undoped GaN barrier layers 42 each having a thickness of6 nm are alternately stacked to each other. The oriented plane of thequantum-well is the (1,0,−1,0) plane. Here, the composition of the welllayer 41 is set such that the displacement of the lattice constant fromthe lattice constant in the unstrained state becomes e_(a)=1.0%,e_(c)=1.1%, and a biaxial compressive strain is applied.

Next, the stacked structure on the substrate 1 is annealed at atemperature ranging from 400 to 800° C. for 20 to 60 min for activatingthe p-type layers. Then, to form an n-type electrode, as shown in FIG.8, part of the grown semiconductor layers are subjected to dry-etching,to expose the n-type GaN buffer layer 3. Subsequently, a metal film madefrom such as Au or Al is formed, followed by patterning, to form both ap-side electrode 10 and an n-side electrode 11. In general, a pluralityof semiconductor laser devices are formed on one wafer, and accordingly,the stacked structure on the substrate 1 is cleaved into resonators eachhaving a length of 800 μm along the (0001) plane, to manufacturesemiconductor laser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 60 mA at room temperatureand the oscillation wavelength was about 420 nm.

<Fifth Embodiment>

FIG. 19 is a structural sectional view, taken from a plane crossing anoptical axis, showing a semiconductor laser device in this embodiment.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on an n-type SiCsubstrate 51 with the (1,0,−1,0) plane taken as the principle planethereof, a low temperature buffer layer 2 made from n-type GaN, a hightemperature buffer layer 3 made from n-type GaN, an n-typeAl_(0.2)Ga_(0.8)N cladding layer 4, an n-type GaN optical guide layer 5,a GaInN multi-quantum well active region 6, a p-type GaN optical guidelayer 7, a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaNcap layer 9. Each of these layers (and region) is formed by epitaxialgrowth by the usual metal organic vapor phase epitaxial growth method.The thickness of each layer is set at 0.05 μm (low temperature bufferlayer 2), 3 μm (high temperature buffer layer 3), 0.3 μm (n-typecladding layer 4), 0.1 μm (n-type optical guide layer 5), 0.1 μm (p-typeoptical guide layer 7), 0.3 μm (p-type cladding layer 8), and 0.5 μm(cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.8)In_(0.2)N well layers 61 each having a thickness of 5 nm andthree pieces of n-doped Ga_(0.95)In_(0.05)N barrier layers 62 each beingdoped with an n-type impurity at the density of 1.6×10¹⁹ cm⁻³ and havinga thickness of 7 nm are alternately stacked to each other.

Here, the density of the impurity doped in the barrier layer 62 is setsuch that the density of electrons released from the impurity andlocally present in the well layer becomes 3.0×10¹⁹ cm⁻³. The orientedplane of the quantum-well is the (1.0,−1.0) plane. Here, the compositionof the well layer 61 is set such that the displacement of the latticeconstant from the lattice constant in the unstrained state becomese_(a)=1.0%, e_(c)=1.1%, and a biaxial compressive strain is applied. Thebiaxial strain means a strain caused by a stress applied to crystallattices of different kinds of crystal layers resulting from adifference between both crystal lattices at an interface at which thedifferent kinds of crystal layers are bonded to each other, which strainmay be simply called “strain” or “lattice strain”. A difference ΔEg inband gap energy between the cladding layer and quantum-well layer is setat 0.8 eV. Next, the stacked structure on the substrate 1 is annealed ata temperature ranging from 400 to 800° C. for 20 to 60 mm for activatingthe p-type layers.

Subsequently, a metal film made from Au or Al is formed, followed bypatterning, to form both a p-side electrode 10 and an n-side electrode11. The stacked structure on the substrate 1 is then cleaved intoresonators each having a length of 800 μm along the (0001) plane, tomanufacture semiconductor laser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 60 mA at room temperatureand the oscillation wavelength was about 420 nm.

<Sixth Embodiment>

FIG. 20 is a structural sectional view, seen from a plane crossing anoptical axis, showing a semiconductor laser device in this embodiment.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on a sapphire substrate31 with the (1,0,−1,2) plane taken as the principle plane thereof, a lowtemperature buffer layer 72 made from n-type AlN, a high temperaturebuffer layer 73 made from n-type AlN, an n-type Al_(0.2)Ga_(0.8)Ncladding layer 4, an n-type GaN optical guide layer 5, a GaInNmulti-quantum well active region 6, a p-type GaN optical guide layer 7,a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaN cap layer9. Each of these layers is formed by epitaxial growth by the usual metalorganic vapor phase epitaxial growth method. The thickness of each layeris set at 0.05 μm (low temperature buffer layer 2), 3 μm (hightemperature buffer layer 3), 0.3 μm (n-type cladding layer 4), 0.1 μm(n-type optical guide layer 5), 0.1 μm (p-type optical guide layer 7),0.3 μm (p-type cladding layer 8), and 0.5 μm (cap layer 9)

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.8)In_(0.2)N well layers 81 each having a thickness of 4 nm andthree pieces of undoped GaN barrier layers 82 each having a thickness of6 nm are alternately stacked to each other. The oriented plane of thequantum-well is the (1,0,−1,2) plane. Here, the composition of the welllayer 81 is set such that the displacement of the lattice constant fromthe lattice constant in the unstrained state becomes e_(a)=0.1%,e_(c)=1.6%, and a biaxial compressive strain is applied. Next, thestacked structure on the substrate 1 is annealed at a temperatureranging from 400 to 800° C. for 20 to 60 min for activating the p-typelayers.

Then, to form an n-type electrode, part of the grown semiconductorlayers are subjected to dry-etching, to expose the n-type AlN bufferlayer 73. Subsequently, a metal film made from Au or Al is formed,followed by patterning, to form both a p-side electrode 10 and an n-sideelectrode 11. The stacked structure on the substrate 1 is cleaved intoresonators each having a length of 800 μm along the (−1,2,−1,0) plane,to manufacture semiconductor laser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 60 mA at room temperatureand the oscillation wavelength was about 420 nm.

<Seventh Embodiment>

FIG. 21 is a structural sectional view, seen from a plane crossing anoptical axis, showing a semiconductor laser device in this embodiment.

The semiconductor laser device having a multi-quantum well structure inthis example is formed by sequentially stacking, on a sapphire substrate1 with the (0001) plane taken as the principle plane thereof, a lowtemperature buffer layer 72 made from n-type AlN, a high temperaturebuffer layer 73 made from n-type AlN, an n-type Al_(0.2)Ga_(0.8)Ncladding layer 4, an n-type GaN optical guide layer 5, a GaInNmulti-quantum well active region 6, a p-type GaN optical guide layer 7,a p-type Al_(0.2)Ga_(0.8)N cladding layer 8, and a p-type GaN cap layer9. Each of these layers is formed by epitaxial growth by the usual metalorganic vapor phase epitaxial growth method. The thickness of each layeris set at 0.05 μm (low temperature buffer layer 2), 3 μm (hightemperature buffer layer 3), 0.3 μm (n-type cladding layer 4), 0.1 μm(n-type optical guide layer 5), 0.1 μm (p-type optical guide layer 7),0.3 μm (p-type cladding layer 8), and 0.5 μm (cap layer 9).

The multi-quantum well active region 6, as shown on the enlarged scale,has a multi-quantum well structure in which three pieces of undopedGa_(0.8)In_(0.2)N well layers 91 each having a thickness of 4 nm andthree pieces of undoped GaN barrier layers 92 each having a thickness of6 nm are alternately stacked to each other. The oriented plane of thequantum-well is the (1,1,−2,4) plane. Here, the composition of the welllayer 91 is set such that the displacement of the lattice constant fromthe lattice constant in the unstrained state becomes e_(a)=1.0%,e_(c)=1.6%, and a biaxial compressive strain is applied. Next, thestacked structure on the substrate 1 is annealed at a temperatureranging from 400 to 800° C. for 20 to 60 min for activating the p-typelayers. Then, to form an n-type electrode, part of the grownsemiconductor layers are subjected to dry-etching, to expose the n-typeAlN buffer layer 73. Subsequently, a metal film made from Au or Al isformed, followed by patterning, to form both a p-side electrode 10 andan n-side electrode 11. The stacked structure on the substrate 1 iscleaved into resonators each having a length of 800 μm along the(1,−1,0,0) plane, to manufacture semiconductor laser devices.

In the semiconductor laser device thus obtained, continuous oscillationwas realized at a threshold current of about 60 mA at room temperatureand the oscillation wavelength was about 420 nm.

<Eighth Embodiment>

FIG. 22 is a perspective view showing a semiconductor laser deviceaccording to a eighth embodiment of the present invention, and FIGS. 23Ato 23D are sectional views, seen along the direction crossing theoptical axis, illustrating the manufacturing process of thesemiconductor laser device. Hereinafter, the semiconductor laser deviceand its manufacturing process according to the present invention will bedescribed with reference to these figures.

As shown in FIG. 23A, a buffer layer 52 (GaN, 0.1 μm), an n-type contactlayer 53 (GaN, 2 μm), an n-type cladding layer 54 (AlGaN, 1 μm), astrained quantum-well active layer 55 [(five layers of GaN (3 nm) andfive layers of Al_(0.4)Ga_(0.6)N (5 nm) are alternately stacked], ap-type cladding layer 56 (AlGaN, 1 μm), and p-type contact layer 57(GaN, 0.3 μm) are sequentially formed on a GaN substrate 51 having the(1,0,−1,2) plane by the metal organic vapor phase epitaxial growth(MOVPE) method. Here, the crystal growth direction on the substrate isinclined 43° from the c-plane, that is, (0001) plane. In other words,the crystal growth plane becomes the R-plane, that is, the (1,0,−1,2)plane.

Next, a p-type electrode 58 is formed on the p-type contact layer 57(see FIG. 23B). Then, the stacked layers are removed by usual etchingfrom part of the surface of the p-type contact layer 57 to the n-typecontact layer 53 (see FIG. 23C), to form an n-type electrode 59 on then-type contact layer 53 (see FIG. 23D). Finally, the stacked structureis separated into chips (laser resonators) in such a manner that an endsurface 60 of the laser resonator becomes the (1,2,−1,0) plane, toobtain semiconductor laser devices. FIG. 22 is the perspective viewshowing the finished semiconductor laser. In addition, from theviewpoint of practical use, the semiconductor laser device may befurther processed by, for example, forming a protective film forprotecting the light emission end surface.

The semiconductor laser device in this embodiment is capable of emittingblue color light oscillating at a threshold current of 40 mA.

In addition, a semiconductor laser device may be obtained by forming anactive layer having a strained quantum-well structure through crystalgrowth on the following substrate. Such a semiconductor laser devicealso exhibits the same effect.

sample inclination crystal No. substrate angle growth plane No. 1 (1, 0,−1, 3) GaN 32° (1, 0. −1, 3) No. 2 (1, 0, −1, 4) GaN 25° (1, 0. −1, 4)

The present invention may be effectively applied to structures otherthan that described in the sixth embodiment. For example, the substratemay be made from not only GaN but also another material having thewurtzite-type crystal structure, for example, sapphire, spinel, SiC,ZnO, MgO, MnO, SiO₂, or AlN.

<Ninth Embodiment>

This embodiment will be described with reference to FIG. 22 and FIGS.23A to 23D.

A buffer layer 52 (GaN doped with Mg, 0.1 μm), an n-type contact layer53 (GaN, 2 μm), an n-type cladding layer 54 (AlGaN, 1 μm), a strainedquantum-well active layer 55 [(five layers of In_(0.2)Ga_(0.8)N (3 nm)and five layers of GaN (5 nm) are alternately stacked], a p-typecladding layer 56 (AlGaN, 1 μm), and p-type contact layer 57 (GaN, 0.3μm) are sequentially formed on a sapphire substrate 51 having the (0001)plane by the metal organic vapor phase epitaxial growth (MOVPE) method(see FIG. 22A). Here, the crystal growth direction on the substrate isinclined 39° from the c-plane, that is, (0001) plane. Accordingly, thecrystal growth plane becomes the (1,1,−2,4) plane.

Next, like the eighth embodiment, a p-type electrode 58 is formed on thep-type contact layer 7, and the stacked layers are removed by usualetching from part of the surface of the p-type contact layer 57 to then-type contact layer 53 (see FIG. 23C), to form an n-type electrode 59at a specific position on the n-type contact layer 53. Finally, thestacked structure is separated into chips (laser resonators) in such amanner that an end surface 60 of the laser resonator becomes the(1,−1,0,0) plane, to obtain semiconductor laser devices. FIG. 22 is theperspective view showing the finished semiconductor laser device. Inaddition, from the viewpoint of practical use, the semiconductor laserdevice may be further processed by, for example, forming a protectivefilm for protecting the light emission end surface.

The semiconductor laser device in this embodiment is capable of emittingblue color light oscillating at a threshold current of 50 mA.

The present invention can be applied not only to the device structuresdescribed in the above embodiments but also to various kinds ofsemiconductor laser devices such as a distribution feedback type laser,Bragg reflection type laser, wavelength variable laser, externalresonator mounted laser, and surface emission laser.

The gallium nitride based semiconductor material is not limited to thathaving the above composition but may be selected from materials having astructural formula expressed by AlxGayIn1−x−yN (0≦x<1, 0<y≦1, 0<x+y≦1),wherein the values x and y may be set such that the band gap energy ofthe active layer becomes smaller than the band gap energy of thecladding layer. Further, in the present invention, there may be used amaterial in which part of N of the above structural formulaAlxGayIn1−x−yN is substituted for As or P.

INDUSTRIAL APPLICABILITY

The optical information processing equipment of the present inventioncan be applied to optical information processing equipment adoptingphotomagnetic recording or phase transition recording method. The lightsource of the present invention can be applied to the above opticalinformation processing equipment. The semiconductor light emittingdevice of the present invention can be applied to the above opticalinformation processing equipment and further used as a light sourceallowing emission of blue light, blue-violet light and violet light.

What is claimed is:
 1. A semiconductor light emitting device comprising:at least a strained quantum-well active layer region made from awurtzite type semiconductor material, wherein a crystal growth directionof said strained quantum-well active layer region is parallel to an axiswhich is inclined 5° to 70° from the [0001] axis (c-axis), wherein saidstrained quantum-well active layer region constitutes part of asemiconductor laser resonator, and wherein an end surface of saidsemiconductor laser resonator is parallel to a plane containing both thecrystal growth axis of said strained quantum-well active layer regionand the [0001] axis (c-axis).
 2. A semiconductor light emitting deviceaccording to claim 1, wherein said end surface of said semiconductorlaser resonator is equivalent to the (−1,2,−1,0) plane.
 3. Asemiconductor light emitting device according to claim 1, wherein saidend surface of said semiconductor laser resonator is equivalent to the(−1,0,31 1,0) plane.
 4. A semiconductor light emitting devicecomprising: at least a strained quantum-well active layer region madefrom a wurtzite type semiconductor material, wherein a crystal growthdirection of said strained quantum-well active layer region is parallelto an axis which is inclined 10° to 60° from the [0001] axis (c-axis),wherein said strained quantum-well active layer region constitutes partof a semiconductor laser resonator, and wherein an end surface of saidsemiconductor laser resonator is parallel to a plane containing both thecrystal growth axis of said strained quantum-well active layer regionand the [0001] axis (c-axis).
 5. A semiconductor light emitting deviceaccording to claim 4, wherein said end surface of said semiconductorlaser resonator is equivalent to the (−1,2,−1,0) plane.
 6. Asemiconductor light emitting device according to claim 4, wherein saidend surface of said semiconductor laser resonator is equivalent to the(−1,0,−1,0) plane.
 7. A semiconductor light emitting device comprising:at least a strained quantum-well active layer region made from awurtzite type semiconductor material, wherein a crystal growth directionof said strained quantum-well active layer region is parallel to an axiswhich is inclined 20° to 55° from the [0001] axis (c-axis), wherein saidstrained quantum-well active layer region constitutes part of asemiconductor laser resonator, and wherein an end surface of saidsemiconductor laser resonator is parallel to a plane containing both thecrystal growth axis of said strained quantum-well active layer regionand the [0001] axis (c-axis).
 8. A semiconductor light emitting deviceaccording to claim 7, wherein said end of surface of said semiconductorlaser resonator is equivalent to the (−1,2,−1,0) plane.
 9. Asemiconductor light emitting device according to claim 7, wherein saidend surface of said semiconductor laser resonator is equivalent to the(−1,0,−1,0) plane.